1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a hydro dynamic bearing device. In particular, the present invention relates to a method and an apparatus for manufacturing a hydro dynamic bearing device to be used in a spindle motor equipped in an information technology device such as a magnetic disk device (e.g., HDD or FDD), an optical disk device (e.g., CD-ROM, CD-R/RW, or DVD-ROM/RAM, and an optical magnetic disk device (e.g., MD or MO), a scanner motor equipped in a copying machine, a laser printer (LBP), a barcode reader, or the like, or a small-sized motor equipped in an electrical equipment such as an axial fan.
2. Description of the Related Art
As is generally known in the art, each kind of the motors listed above have been promoted to be provided at lower cost, driven at higher speed and more quiet, and so on in addition to attain a high rotational accuracy. As one of factors that define these required performances, a bearing supporting spindle of the motor has been increasingly valued. In recent years, therefore, as such a kind of the bearing, the use of a hydro dynamic bearing having excellent characteristics that serve a request for the above performance has been studied, and such a hydro dynamic bearing has been developed in a quest to put it to practical use.
For instance, a hydro dynamic bearing device to be built in a spindle motor of a disk device such as a hard disk drive (HDD) comprises a radial bearing part for rotatably retaining an axial member in a non-contacting manner in the radial direction and a thrust bearing part rotatably retaining the axial member in a non-contacting manner in the thrust direction. As a bearing part of each of them, a hydro dynamic bearing device having a groove (a hydro dynamic pressure generating groove) for the generation of hydro dynamic pressure on its bearing surface is used.
In this case, the hydro dynamic pressure generating groove of the radial bearing part is formed in the inner peripheral surface of the bearing member or the housing, or formed in the outer peripheral surface of the axial member. On the other hand, in the case of using an axial member having a flange part, the hydro dynamic pressure generating groove of the thrust bearing part is formed in each of the opposite end faces of the flange part or the surface (e.g., the end face of the bearing member or the bottom surface of the housing) facing to such an end face.
In addition, when an axial member having no flange part is used, there may be adapted a structure that retains the end face of the axial member by a thrust plate attached on the bottom surface of the housing without forming a hydro dynamic pressure generating groove in the bearing surface as a thrust bearing part.
In each of these hydro dynamic bearing devices, furthermore, the axial member is projected outward from the opening part of the housing. For sealing the lubricating oil filled in the housing, a sealing part that covers the periphery of the axial member through a small clearance is provided on the opening part of the housing.
In the final stage of manufacturing such a kind of the hydro dynamic bearing device, a hydro dynamic bearing device is assembled without lubrication and is then dipped in lubricating oil in a vacuum tank, followed by opening the chamber to atmospheric pressure to fill the inner space of the housing with the lubricating oil.
Subsequently, the hydro dynamic bearing device is removed from the vacuum tank after filling with the lubricating oil. In this case, however, a large amount of the lubricating oil adhere to various parts of the removed hydro dynamic bearing device, for example to the outer peripheral surface and the outer bottom surface of the housing, the outer end face of the sealing part, and the outer end face of the axial member.
Then, the lubricating oil adhering to each of these parts is wiped off at a subsequent step. Conventionally, the wiping has been performed using a sheet or film formed of a resin material based on polyethylene, polyvinyl chloride, polyvinylidene chloride, or the like, specifically such as Saran Wrap (trade name) manufactured by Asahi Kasei Corporation.
However, when the lubricating oil is wiped off using the resin film or the like as described above, the wiping is a complicated and very hard work that takes much time to complete, while becoming too burdensome for the worker. Consequently, there is a possibility of decreasing the working efficiency. In this method, furthermore, there is a tendency that it becomes difficult to properly wipe the lubricating oil off from each of the above parts.
That is, the housing of this kind of the hydro dynamic bearing device typically comprises a lateral part having an outer peripheral surface and a bottom part having an outer bottom surface, which are separately provided and fixed together with caulking or the like. Therefore, an uneven spot or a step is found in the joining area between these components. Therefore, when the lubricating oil adhering to the housing is wiped off using the resin film or the like as described above, the uneven spot or the like stands in the way of wiping the lubricating oil to prevent a smooth wiping-off operation. In addition, there is a possibility of remaining the lubricating oil still in a recessed part. As a result, it is concerned about the quality of the product which may decrease.
In addition, on the opening part side of the housing, the axial member is protruded outward by passing through a through hole formed on the center of the sealing part, resulting in that the axial member stands in the way of wiping off the lubricating oil around the axial member using the resin film or the like as described above. Consequently, there is a possibility that it becomes difficult to wipe off the lubricating oil on the outer end face of the sealing part and the outer end face of the axial member, and it is difficult to prevent the lubricating oil being remained in place.
Meanwhile, the wiped hydro dynamic bearing device (or one being subjected to rough wiping) is heated up to an upper limit temperature of the operation to thermally expand the lubricating oil filled in the internal space of the housing, removing an excess amount of the lubricating oil therefrom.
This kind of the treatment brings down the oil level of the lubricating oil at room temperature as much as sufficient to keep it an appropriate level to prevent the lubricating oil from being leaked due to the thermal expansion of the oil when the hydro dynamic bearing drive is operated at an upper limit temperature of the operation. In addition, an excess amount of the lubricating oil being overflowed at this time (i.e., a small amount of the lubricating oil) is also subjected to the wiping-off treatment.
Under present circumstances, the oil level of the lubricating oil in the hydro dynamic bearing device is adjusted such that an increased part of the oil level concurrently with the thermal expansion is absorbed with an oil-absorbing member such as a cotton swab at the time of heating the lubricating oil after the lubricating oil has been filled in the inner space of the housing.
However, the conventional adjustment of oil level is performed using such a naturally-occurring absorbing action, i.e., a capillary phenomena or the like, and in this case, the absorbing performance and the absorbing function of the above action depend on the characteristics of an absorber such as a cotton material, while it is also influenced by the degree of practiced experiences of a worker. Therefore, the management of the oil-level height of the lubricating oil in the housing may be difficult since variations in absorbing volume may occur.
When the above variations cause a shortage of the lubricating oil in the housing, several troubles occur, for example insufficient lubrication of the bearing device. Consequently, the hydro dynamic bearing device may deteriorate earlier than expected. In addition, when the amount of the lubricating oil in the housing is excessive, there is a possibility of causing a leakage of the lubricating oil to the outside when the hydro dynamic bearing device is operated at an upper-limit temperature of the operation.